SPCR

My brother has a Lian Li PC-A71 which is pretty quiet, only some faint fan noise can be heard. Those fans really are no problem, they are quiet enough, it's the hdd seek noise that's too loud to his (and my) likings. One, or all the drives are making a muffled 'dunk' noise, no rattles, just a solid 'dunk' seek noise.

What can he do to make his drives more quiet? We looked into the elastic suspension method but with 7 drives there's not enough 5-1/4" bays, and he doesn't want to cut away the original 'soft mount' rails.

7 Drives? That's a lot. I'd suspend all I could elastically, slower spinning drives with 5400 and 5900 RPM tend to have milder seeknoise and vibrations so if you have to mount a drive with stock-rubber solution, it should be a slow drive.
Best is to suspend them all off course but if that is not an option then get rid of the worst seeking drives by suspending them.

I dampened my car a few years ago with those (i think) 'bitumen' mats, I have a few boxes of those mats left, would those help with reducing hdd seek noise?
I have 2 varieties: 2,5mm (5kg/m²) and 4mm (8kg/m²).

Most of the drives are Samsung Spinpoints, there's also a Seagate 7200.12 IIRC.

7 drives? sounds like a losing battle to me.
put them out of earshot in a remote nas. that will also allow for less fan noise since the case will run way cooler with 6 less drives and you can turn down the fan(s).

Well, he ordered Scythe slipstreams for the case (current system temp is ~27°C).
The system is really quiet actually, we turned the fans down once and then the only audible noise is the seek noise. We can't even hear the drives spinning.
He doesn't want a SPCR quiet system, but only quiten those seek noises to a more acceptable level. In fact they are already more quiet than the off the shelf Compaq my dad has...

If seek noise is your only concern, you could try enabling acoustic management on the drives. As long as it's supported, the seek noise is reduced but some performance is lost as well.

If you're under Windows, Notebook Hardware Control and Crystal Disk Info both have pretty GUIs. If you're not scared by the command line, hdparm does the job too (though I never tested that feature under Win)